1. Field of the Invention
The present invention relates to a projection lens and, more particularly, to a projection lens for projecting an enlargement of an image appearing on a cathode ray tube (CRT) on a screen to provide a clear image through correction of optical aberrations using a low number of lens elements.
2. Background of the Related Art
Related projection lenses for a wide screen television are disclosed in U.S. Pat. No. 4,300,817 to Betensky (Betensky '817), U.S. Pat. No. 4,384,081 to Kubo, et. al. (Kubo '081), and U.S. Pat. No. 4,526,442 to Betensky, et. al (Betensky '442).
The related projection lenses, which are generally designed for a CRT, generally include three lens units. The three lens units are generally a first lens unit with at least one aspheric surface disposed at the foremost end of a screen for correcting aberrations, a second lens unit having a biconvex lens element of positive power, and a third lens unit that functions as a field flattener and that has a concave lens element to correct a Petzval curvature of the lens.
The lens units, which are also called lens groups, each have specific optical functions and are disposed such that the one or more individual optical elements that make up the lens unit are spaced from each other by a predetermined distance. The specific optical functions of each lens unit or lens group in the overall lens system are performed by one or more lens elements. The number of lens elements in each lens unit or lens group depends on the required optical information or required optical functions.
Other factors that influence the number of lens elements in each lens unit include the desired optical performance of the lens, cost constraints, and size constraints. A lens unit, or a lens group, is defined as one or more lens elements that enable an optical function in the overall lens system.
In Betensky '817, a second lens unit is disclosed with one biconvex lens element, and an equivalent focal length (EFL) of the overall lens system disclosed is 127 mm or greater. In Kubo '081, a second lens unit includes a biconvex lens and is used in a direct projection lens for a CRT with a diameter of 5 inches. The second lens unit is designed to reduce the equivalent focal length of the overall projection lens to 85 mm.
In Betensky '442, folding means are inserted between a first lens unit and a second lens unit to fold the optical axis. Also, the equivalent focal length of the overall lens system is 126 mm or less, which makes the lens system suitable for a CRT screen with a diameter of 5 inches.
A projection television (TV) set requires a cabinet with a large volume in order to accommodate a large screen. A recent trend has been to minimize the volume of the cabinet while still accommodating a large screen and maintaining a clear image quality. In order to reduce the volume of the cabinet, methods have been proposed for reducing the equivalent focal length of the overall projection lens system. If the equivalent focal length of the projection lens is reduced, a field angle of the projection lens is increased.
The equivalent focal length of the projection lens has a functional relationship with the overall distance between the CRT and the screen as shown in equation (1) below: ##EQU1## where OL is the overall length between an object and an image, EFL(1+1/M) is a distance between the object and a second principal point of the lens, EFL(1+M) is a distance between the object and a first principal point of the lens, and M is the magnification of the lens system, i.e., the ratio of the height of the image to the height of the object.
As illustrated in equation (1), it is necessary to reduce the equivalent focal length or to increase the field angle of the lens in order to reduce the overall distance between the CRT and the screen. However, reducing the equivalent focal length or increasing the field angle makes it difficult to correct for optical aberrations.
Another method involves enlarging a space between a fluorescent screen of the CRT and a third lens unit of the projection lens. However, this method requires a liquid coolant, for cooling a fluorescent plane of the CRT, a coupling material, and a housing for containing the liquid coolant. Furthermore, if the fluorescent plane of the CRT is enlarged, the concave-shaped third lens unit must have high negative power. As a result, the optical power of the convex-shaped second lens unit must be increased.
Since most CRT fluorescent screens include a convex lens to increase the brightness of corner portions of the CRT system, the optical power of the third lens unit must be reduced to correct the field curvature. However, it is difficult to obtain a margin angle required to correct astigmatism and distortion with a second lens unit that has one biconvex element.
U.S. Pat. No. 4,348,081 to Betensky (Betensky '081) discloses techniques for correcting astigmatism and distortion. As disclosed in Betensky '081, better correction of aberrations can be achieved by dividing the optical power of the second lens unit at a short focal length. However, if the optical power of the second lens unit is merely divided into two, in order to obtain the margin angle, the available use angle of the projection lens system exceeds 72 degrees. As a result, it is difficult to obtain sufficient resolution and contrast.
Betensky '817 teaches that a large sized projection TV for home use can be manufactured. Accordingly, much attention has been given to the development of a projection lens that has a wide field angle, and that can be manufactured at low cost. Such a projection lens can reduce the size of a TV system, while maintaining or increasing the size of the viewing screen.
U.S. Pat. No. 4,948,327 to Hirata et. al. (Hirata '237) and U.S. Pat. No. 4,697,892 to Betensky (Betensky '892) disclose projection lenses that are designed to reduce the focal length of the overall lens by dividing the complex second lens unit into two or more lens elements. The techniques taught in Hirata '237 and Betensky '892 are currently applied to wide screen projection lens systems with focal lengths of 80 mm or less. These projection lens systems have a high positive optical power due to the second lens unit, and have an increased field angle so that correction of field curvature aberration can be easily achieved. This results in good optical performance.
However, in the projection lens systems disclosed in Hirata '237 and Betensky '892, the second lens unit must have convex element with a large diameter. In addition, the second lens unit must have a significantly long front vertex distance (FVD) for appropriate correction of field curvature aberration and astigmatism. The front vertex distance is the distance between an image of the first lens unit and the plane of the CRT.
U.S. Pat. No. 4,801,196 to Betensky (Betensky '196), discloses a projection lens that includes a first lens unit comprised of a single lens element with two aspheric surfaces and an overall positive meniscus shape, a second positive lens unit, and a third lens unit with a strongly negative concave surface facing an image end. The first lens unit is of positive optical power at the optical axis of the lens. However, due to the aspheric power of the surfaces, the positive optical power of the first lens unit decreases with increasing radial distance from the optical axis, and may become strongly negative at or closely adjacent to the clear aperture of the first lens unit.
The strong negative power of the third lens unit contributes to correction of the Petzval sum of the other lens elements. The strongly concave surface may be made aspheric to also correct for residual astigmatism and field curvature of the lens. The second lens element provides the majority of the positive power of the lens system and some astigmatism correction. The first lens unit must then correct the aperture-dependent aberrations, particularly spherical aberration and coma The lens systems described in Betensky '196 are very compact due to the close spacing between the first and second lens units. These lens systems may have a field angle as great as 73 degrees, while comprising only three elements.
The lens elements disclosed in Betensky '817 are made of acrylic material on which aspheric surfaces may be easily formed. However, the refractive index of acrylic varies significantly with temperature. This leads to a change in focal length of the acrylic lens elements which, in turn, can lead to defocusing of the overall lens system.
A method has been disclosed for designing a mount and a barrel for the lens system, using a bimetallic plate or other means, that shifts the position of the lens system relative to the CRT in order to maintain the focus of the lens system during temperature changes.
Kubo '081 addresses the temperature problem by minimizing the variation of the optical power with changing temperature. This is accomplished by using a glass material to make the second lens unit or the power lens. A problem with this method is the high expense involved in forming an aspheric surface on glass.
To solve the expense problem, a hybrid lens system has been disclosed that comprises a power unit made of a glass material, and an additional acrylic corrector with one or more aspheric surfaces adjacent to the second lens unit or the power unit. However, it is difficult to provide a hybrid lens system with a wide field angle and short length.
U.S. Pat. No. 4,776,681 to Moskovich (Moskovich '681) addresses the hybrid lens problem. FIG. 1 shows one of the embodiments disclosed in Moskovich '681, in which aspheric surface lenses made of a plastic material are used. However, as illustrated in FIGS. 2A-2C, aspheric surface lenses made of plastic cannot achieve sufficient correction of chromatic aberration. Further, the lenses disclosed in Moskovich '681 have been designed and developed for the peak wavelength of a green CRT, i.e., the e-line (546.0 nm). Since such lenses are also applied to both a red CRT and a blue CRT, the lenses do not perform optimally.
As illustrated in the longitudinal spherical surface aberration plots of FIG. 2A, a peak wavelength f-line (490.0 nm) of the blue CRT, a peak wavelength d-line (590.0 nm) of the red CRT, and a peak wavelength e-line (546.0 nm) of the green CRT are out of the center of aberration. This results in deterioration of the optical lens system performance. As illustrated in the field curvature aberration plots of FIG. 2B, it is difficult to correct coma aberration and field curvature aberration due to sagittal (S) and tangential (T) field curvature in the center of the aberration distance. This results in distortion, as illustrated in FIG. 2C.
FIGS. 3A-3C show spectral characteristics of green, blue, and red CRTs. The spectral characteristics and chromatic aberration occur because each CRT emits light at a peak wavelength and at other wavelengths within its emission band. The overall image quality exhibited by a projection TV depends mainly on the green lens assembly. This is because, as illustrated in FIG. 3A, the green CRT emits light at wavelengths that fall within the blue and red wavelength bands. Thus, the projection lens must correct the chromatic aberration in the blue and red wavelength bands. Since non-corrected chromatic aberration causes deterioration in picture quality and contrast it is necessary to correct the chromatic aberration for High Definition Television (TV) and high definition video projectors.
The respective lens units are denoted by a reference code G and are numbered in series from an image side to an object side of the lens system. The reference code CR denotes the correction lens unit. The lens elements are denoted by a reference code L and are numbered in series from an image side to an object side of the lens system. The surfaces of the respective lens elements are denoted by a reference code S and are numbered in series from an image side to an object side of the lens system. The reference code CS denotes a CRT screen. The reference code OC denotes a liquid optical coupler between the CRT screen CS and the lens system.
The first lens unit G1 includes a meniscus-shaped lens element of positive optical power. The first lens unit G1 has at least one aspheric surface defined by equation (2) below: ##EQU2## where x is the surface sag at a semi- aperture distance y from the axis A of the lens element, C is a surface curvature of the lens element at the optical axis, K is a conic constant, and D, E, F, G, H, and I are aspheric surface coefficients.
The first lens unit G.sub.1 is more convex than the second lens unit G.sub.2, and thus consists of a meniscus-shaped lens element having a small space. The second lens unit G.sub.2 includes a biconvex lens element having biconvex surfaces S.sub.3 and S.sub.4, and is made of a glass material to minimize the variation of its optical power with changing temperature. The third lens unit G.sub.3 acts as a field flattener and has the same Petzval curvature as the first lens unit G.sub.1 and the second lens unit G.sub.2. The third lens unit G.sub.3 has a negative optical power, and includes a spherical surface and an aspheric surface.
The distance d.sub.12 between the first lens element L.sub.1 of the first lens unit G.sub.1 and the second lens element L.sub.2 of the second lens unit G.sub.2 is an important factor in correcting the field curvature aberration. Specifically, the distance d.sub.12 has to satisfy equation (3) below: EQU 0.10&lt;.vertline.d.sub.12 /f.sub.3 .vertline.&lt;0.48, (3)
where f3 is the focal length of the third lens unit G.sub.3.
If .vertline.d.sub.12 /f.sub.3 .vertline. is 0.10 or less, the field curvature is overcorrected and an image does not appear. On the other hand, if .vertline.d.sub.12 /f.sub.3 .vertline. exceeds 0.48, it become difficult to correct the field curvature aberration, and an image does not appear. Furthermore, as the field angle of the lens system gets smaller, the astigmatism of the lens system gets higher. Such astigmatism can be corrected by correcting the optical aberration in the second lens unit G.sub.2.
The coma aberration, the astigmatism, and the optical aberration in the second lens unit G.sub.2 should be corrected in such a manner that the lens element L.sub.1 of the first lens unit G, has two aspheric surfaces S.sub.1 and S.sub.2. The optical power of lens element L.sub.1 is positive at the optical axis, but goes down with increasing radial distance from the optical axis. The rate of change of the optical power along the radial distance of the lens element L.sub.1 is dependent on the optical aberration being corrected. The spherical optical power Kca/ka of the first lens element L.sub.1 around its effective aperture, or at the optical axis thereof, is disclosed in U.S. Pat. No. 4,685,774 to Moskovich (Moskovich '774).
The positive optical power at the optical axis is reduced, depending on the height Y from the optical axis, to negative optical power with an absolute value of at least 2.5 times the optical power at the optical axis. The spherical optical power Ky of lens element L.sub.1 can be expressed as follows: EQU K.sub.Y =(n-1)(C.sub.1Y -C.sub.2Y), (4)
where n is the optical power of lens element L.sub.1, C.sub.1Y is the curvature of the first lens surface S.sub.1 between the optical axis A and the height Y, and C.sub.2Y is the curvature of the second lens surface S2 between the optical axis A and the height Y.
However, such a lens element has several problems in that it exhibits the longitudinal spherical aberration shown in FIG. 2A. That is, the peak wavelength F-line (490.0 nm) of the blue CRT, the peak wavelength d-line (590.0 nm) of the red CRT, and the peak wavelength e-line (546.0 nm) of the green CRT are away from the center of the aberration distance. As a result, significant chromatic aberration occurs. In addition, as illustrated in FIG. 2B, the sagittal field curvature S and the tangential field curvature T in the center of the aberration distance prevents the correction of the coma aberration and the correction of the field curvature aberration, and causes the distortion illustrated in FIG. 2C. Since such chromatic aberration deteriorates the resolution and contrast of the lens system, the lens system of FIG. 1 is not suitable for HDTV and video projectors.